1. Field of the Invention
The present invention relates in general to underwater communication and, in particular, to underwater communication in the presence of multipath interference.
2. Description of Prior Art
Tracking, or locating underwater vehicles in real time, is a prime concern in undersea warfare research and testing. Vehicles to be tracked are outfitted with specialized acoustic transmitters and underwater tracking ranges are built by implanting receiving hydrophones on the sea floor. Accurate measurement of travel time of acoustic messages, traveling from the underwater vehicle to the implanted hydrophones, is combined with an effective sound speed to give real-time slant range. Slant range to multiple-implanted hydrophones allows for geometric calculation of vehicle position.
Reliable and effective communication across the acoustic path is the main problem encountered. Temperature variations, reflective surfaces, sea states, and noise (natural and man made) are all dynamic factors that defy precise, real-time characterization, complicating the communications task.
In general, the following are required:
1. Message Validation--Incoming messages must be reliably validated and arrival times measured accurately. Operation at the lowest Signal-to-Noise ratio (or Signal-to-Interference ratio) is required to obtain maximum slant ranges.
2. Message Identity--Validated messages must be identifiable. At any given time the acoustic channel may contain multiple signals that need to be identified as to source.
3. Telemetry--The ability to transmit free-form telemetry across the acoustic channel is desirable.
4. Doppler--For underwater tracking applications, the system must operate at Dopplers produced by high-speed underwater vehicles.
The ability to track underwater vehicles in deep water has existed for some time and deep-water tracking ranges are abundant. Only recently have firm requirements evolved that require the ability to track objects in shallow water where slant ranges, compared to water depth, are very large.
Deep water ranges typically enjoy acoustic paths free from harmful reflective interference. Shallow water implementations usually do not enjoy interference-free acoustic paths. Additionally, in a shallow water tracking range, the underwater vehicles are very near the plane of the implanted hydrophones. In this situation, Z-axis, or depth calculations become indeterminate and if depth is to be known in real time, it must be measured on board the vehicle and telemetered across the acoustic channel. For shallow-water tracking, reliable acoustic telemetry is an absolute requirement.
In shallow water, direct acoustic path propagation seldom exists over any appreciable distance. Long distance transmission of information requires bounce-path propagation. In bounce-path propagation not one but multiple paths exist between the transmitter and receiver. Received signals consist of multiple replicas of the transmitted signal, all overlapping each other and continuing with decaying amplitude.
Constructive and destructive interference of the overlapping signals effectively destroys conventional modulation methods such as On-Off Keying (OOK), Frequency-Shift Keying (FSK), and Phase-Shift Keying (PSK).